The PLC memory system stores a large amount of information about the statuses of all the inputs and outputs. In order to keep track of this large amount of information, the PLC uses a system called addressing.
An address is a label or number that indicates where a certain piece of information is located. This number directs the PLC where to look when it needs to access that piece of information. There are two types of addressing.
A logical address is the address at which a certain item (memory cell, network host, etc) seems to reside according to the application program.
A physical address is a memory address that is represented by binary numbers on the circuitry in order for the processor to access a specific location of memory.